Floating structures used for offshore oil and gas drilling and production are known. One such floating structure is conventional semi-submersible hulls. A conventional semi-submersible hull has a square pontoon structure. The square pontoon structure is coupled to four square shaped columns placed at the four corners of the pontoon structure. Therefore, the pontoon section length is the same as the length separating the columns.
In a conventional semi-submersible hull, the columns do not have strakes. Each column is connected to a deck structure to support topside facilities. A spread mooring or dynamic positioning system is used for station keeping.
Conventional semi-submersible hulls have several limitations. They are subject to large heave, roll and pitch motions. A conventional semi-submersible hull is unable to support steel catenary risers in extreme weather conditions. These steel catenary risers also have fatigue problems in long term operating conditions. Furthermore, a conventional semi-submersible hull is unable to be used for dry tree production applications while undergoing these motions.
There are also known variants of this structure that alter the draft and the column distance of the floating platform. In traditional structures, the length of the pontoon structure is considerably larger than the draft. In an attempt to reduce the effects of motions experienced in extreme and operating weather conditions, structures were developed with an increased draft and/or modified column distance. However, these deep draft variants are still operationally limited.
Another type of known floating structure is an extendable draft platform (EDP). An EDP structure includes a buoyant equipment deck. The buoyant equipment deck is either rectangular or triangular. Column wells are coupled to each corner of the buoyant equipment. On an opposite end, the columns are coupled to a heave plate. In an EDP structure, each of the columns has an upper portion with a diameter that is different from that of a lower portion, which is usually smaller. The columns can move vertically in the column well to adjust the draft.
EDP structures have several limitations. EDP structures are difficult to manufacture and maintain because they use complex, large moving components. Additionally, strong sub-surface currents can cause vortex-induced vibrations (VIV). A structure that has prolonged exposure to VIV can experience fatigue damage to components and is subject to structural failure.
A dual column semi-submersible hull is a known floating structure as well. A dual column semi-submersible hull has a deck structure that is supported by vertical columns arranged in pairs. In these structures, one set of the paired columns is displaced a distance outward from the other set of paired columns. The other set of paired columns is in line with a pontoon structure. The lower ends of this set of vertical columns are connected to the pontoon structures.
The dual column semi-submersible hull, at a much deeper draft, has better performance than a conventional semi-submersible hull at a much shallower draft. However, at the same draft, the dual column semi-submersible hull only marginally improves the motions of a conventional semi-submersible hull. In addition, the dual columns complicate design, fabrication and operation.
There is also a central pontoon semi-submersible floating platform. The central pontoon structure is disposed inboard of the columns, with each of said vertical support columns having a transverse cross sectional shape with a horizontal major axis oriented radially outward from a center point of said hull. However, the vertical wave force on the central pontoon not substantially cancelled by the forces on the columns. This arrangement has adverse effects, and can result in worse vertical motions than a conventional semi-submersible hull at the same draft.
The other known semi-submersible is octabuoy. The draft of octabuoy is substantially greater than the distance between the columns' central axes. The columns have quite large diameter relative to the length of pontoon section, and the pontoon section length is around 2 times column diameter. As a result, the column displacement is a few times greater than the pontoon displacement, and the wave forces on the columns will make a greater contribution than the force on the pontoon. The most preferred draft of octabuoy is at least 60 meters, and the most preferred ratio of draft to the distance between central axes of columns is 1.3 to 1.35. The substantially deep draft required makes it cannot integrate topsides at quaysides because of water depth limitations. Additionally, float over operations near the shore are required. The nonlinear shape and variant cross section of columns also increases fabrication complexity.
Therefore, for the drilling and production of offshore oil and gas, there is a need for a simple floating structure that is subject to minimized environmental forces and platform motions compared with known semi-submersibles.